The present invention relates to a method for controlling the output speed of a fluid friction clutch by regulating the filling ratio by means of a valve located between the work chamber and the reservoir. The invention furthermore pertains to an apparatus for carrying out said process, i.e., a fluid friction clutch having a work chamber and a reservoir, separated by a partition in which a valve actuated by a valve lever is located for regulation of the filling ratio.
Such fluid friction clutches are known, for example, from German Offenlegungsschrift No. 28 04 557 or German 0ffenlegungsschrift No. 28 04 859.
In the fluid friction clutch according to German Offenlegungsschrift No. 28 04 859, the valve lever of the fluid friction clutch is actuated by means of an electromagnet activated as a function of the temperature of the cooling medium. This so-called two-position control (on/off switch) operates in a relatively coarse manner and does not allow, without further assistance, the maintenance of stable intermediate speeds between the minimum and maximum output speeds.
In the fluid friction clutch according to German Offenlegungsschrift No. 28 04 557, the valve lever is actuated by means of a heated expansion element, which is also activated as a function of the cooling medium temperature. Activation by means of an expansion element has the disadvantage, in relation to the control of the output speed of a fluid friction clutch, that the cooling of the expansion element depends on different environmental conditions and therefore takes place in a relatively uncontrolled manner. The control process is thus relatively sluggish, possibly resulting in a delayed clutch response.
A fluid friction clutch is known from European Pat. No. 9415, wherein the valve lever is actuated electromagnetically via the drive shaft, and regulated continuously between an open and a closed position in a cyclical manner. The disadvantage in this so-called cycling process is that a high activating power is required as a consequence of the rapid valve actuation.
Because of the large number of switching cycles, there is the further disadvantage that the cyclically regulated parts of the valve mechanism are exposed to greater mechanical wear, which has a negative effect on the life of these structural elements and thus on that of the clutch.
Finally, German Offenlegungsschrift No. 27 28 901 discloses a method to limit the rotational speed of a fan driven by such a clutch, wherein the clutch is a hydrodynamic clutch with filling ratio regulation. This regulation is effected by means of a clutch control which regulates an additional fluid circuit drive by a pump, with regulation in the clutch control taking place by means of an electromagnetically actuated valve. Such clutches and clutch controls are technically complicated and costly.